My concern would be degradation of efficiency over time as the roof gets dirty. If we're talking about a few percentage points of efficiency, cleanliness will

My concern would be degradation of efficiency over time as the roof gets dirty. If we're talking about a few percentage points of efficiency, cleanliness will make a big difference. I doubt the paint is robust against frequent cleaning. This sounds like it would be much better than cheaper alternatives (like whitewash mentioned by others) for one summer, but just one summer.